This randomized controlled trial will test the long-term efficacy of brief clinician advice and the Pathways to Change expert system interactive computer program for reducing the prevalence of smoking among 3,200 teens who are being seen for routine medical care. Both smokers and nonsmokers, ages 14 to 17 at baseline, will be eligible for this population-based intervention that is highly tailored to the needs of the individual. The primary endpoint will be 30-day smoking prevalence, as measured two years after randomization. Secondary measures will include susceptibility to smoking, stage of smoking acquisition or smoking change, and other tobacco use measures. We will also identify predictors of smoking at follow- and factors that moderate intervention effects. The intervention will capitalize on the teachable moment offered by a primary care physician appointment and the attractiveness of computers and this multimedia interaction program to teens. Others key attributes of this strategy are; 1) a focus on the whole population of adolescents (whether smoking at entry or not); 2) a highly tailored stage based theory-driven intervention; 3) the ease and practicality of the intervention for clinicians; and 4) the relatively low variable cost of delivering the intervention to the teen population seen during routine medical care. Although this study will be conducted in an HMO setting, the intervention would be practical in any primary cre medical care setting that provides care to adolescents.